FLAC is, by definition, loss-less. From a FLAC file you can recreate the original PCM stream, bit-for-bit.
Vorbis (sometimes referred to as just Ogg when you know the codec is for audio-only), the scheme used in the Ogg container to do the compression, is a lossy compressor, same as MP3. From an Ogg Vorbis encoded file you cannot recover the original WAV stream, bit-for-bit, but on playback the claim is it sounds pretty much the same.
My opinion: for long term storage you want lossless compression using a codec with a strong developer backing. FLAC definitely meets that standard. Using a lossless compression scheme means you save storage space, but you can transcode to any future format and know that you're re-encoding an exact copy of your source as it was when it was first ripped. I personally rip my CDs to FLAC and transcode to m4a for use on my iPhone.
Licensing and patent issues, if any
Both FLAC and Ogg Vorbis are patent-free. Both are in the public domain. Both are completely free to use in commercial and non-commercial products.
If you use source code supplied by Vorbis or the FLAC developers it comes with a GPL license. But you can code your own implementation of the specs and not have to release your source code, the spec itself is public domain.
My opinion: both are formats with lots of developer backing and both specs are in the public domain meaning even if all the current developers disappear, if the code gets lost, someone with enough drive can always build a codec from the raw spec at any point in the future if they need it. That's good future-proof-ness that is. There are also GPL implementations currently available so there is code that will remain in the public domain for all time to come, assuming it's never lost, you can always get the codec back.
Decoding FLAC is not very CPU intensive. Akin to uncompressing some data. Vorbis decoding is more intensive due to its lossy nature and the algorithms required to recreate the sound are more involved. But on modern hardware neither format will push the limits of your processing power.
My opinion: I'd say performance is a non-issue when comparing the formats.
is there any reason I should not stick to MP3?
This is somewhat subjective. If your MP3s sound good and you don't mind using a format that's patented and not free to implement than no, there's no good reason to switch. But for archival of data I'm of the mind that non-proprietary compression for space saving coupled with public domain algorithms are the only way to protect yourself from the Beta Syndrome (where you end up with a bunch of stuff on media that can only be played on proprietary gear that is no longer made by the one company that made the stuff to begin with). FLAC is that codec. Ogg Vorbis is a nice alternative to MP3 but lossy codecs, IMO, have no place in long term archival. A rip to FLAC and then a transcode to the flavour-of-the-week lossy codec for use on your cellphone is the best strategy around. Go forth and FLAC my good man.